Wax-based, hot-melt laminates have long been used in the packaging industry. They offer excellent protective barrier properties and thus serve to protect the contents of packages against the loss or gain of moisture vapor or gases.
In addition to performing as a protective barrier film, such laminates also act as adhesives. The adhesive characteristics permit the joining of dissimilar films, foils or papers. In addition, hot melt laminates are important adhesives for bonding difficult to wet substrates or for bonding substrates which are rigid and load bearing.
The prior art hot melt laminates largely consisted of three ingredients--an elastomer (such as a conventional ethylene vinyl acetate copolymer containing about 28 weight percent vinyl acetate units), a tackifier and a wax. The strength of the laminate is measured by its ability to bond two surfaces together. This is controlled not only by the adhesion to the surfaces involved, but also to the cohesive strength of the laminate itself and by the cohesive characteristics of the substrate. The adhesive strength factor depends upon the chemical-mechanical bond that is created between the laminate and the surface of the substrate.
Laminate adhesives are used in several industries, including the food industry. As landfills continue to close and concerns about the environment are increased, interest in development of recyclable laminate adhesives is being renewed.
The recycling of wastepaper from materials containing laminate adhesives proceeds generally by one of three methods. In two of the methods, the adhesive is separated from a water slurry of recycled fibers using either cleaners or screens. When screens are employed, the pulp slurry is passed through a cylindrical screen plate fitted with small holes or fine vertical slots. Particles of the adhesive are separated out and rejected. When cleaners are used, the pulp slurry is pumped into the cleaner to form a spiraling flow. The slurry is thereby subjected to a centrifugal field. A central core is formed in which the particles of adhesives concentrate. A portion of the pulp slurry is extracted from this central core carrying the rejected adhesive particles.
In the third method, a thickened pulp slurry is heated to a temperature, generally ranging from 180.degree. F. to 250.degree. F. It is then subjected to an intensive shearing and/or kneading action. The adhesive particles are broken down to fine sizes, thereby minimizing spotting. Considerable energy is expended, however, in the process.
In the design of modern recycling systems, interest has been centered on those properties of laminates which are useful in the separation of the adhesive particulates from the pulp slurry. Much attention has been paid to the size of the adhesive particulate and its resistance to breakup when the wastepaper is wetted and broken up in a pulper. Pulpers have been specifically designed to minimize breakup of the laminate particulate during the pumping action. The larger particulates are separated from the pulp slurry with screens. To separate the particulates that pass through the screens, the pulp slurry is pumped into a through-flow cleaner wherein the smaller adhesive particulates are removed. If screening and cleaning are insufficient to produce the required cleanliness, the pulp is subjected to dispersion.
Laminate adhesives must be repulpable in order to maximize wastepaper quality. Such laminates typically contain an elastomeric resin and a tackifying resin. In addition, they may contain antioxidants, fillers and plasticizers. The elastomeric resin serves to form the backbone and contributes to the cohesive strength and toughness of the composition. Tackifying resins promote specific adhesion and help the adhesive to wet the substrate. In addition, a wax is added to decrease melt viscosity, increase the crystallinity and control the open and set times of the elastomeric resin.
Microcrystalline waxes, as well as paraffin waxes, are principally used in commercially available laminates. In addition to such waxes, commercially available laminating adhesives further contain a tackifier and a resin. The resin is principally either a conventional ethylene vinyl acetate copolymer (having between 25 and 28 weight percent of vinyl acetate units) or a synthetic rubber. Previous efforts to recycle materials containing laminate adhesives has been largely unsuccessful due to the non-dispersability of waxes and resins contained in such adhesives.
It is most desired to develop a laminating adhesive which is compatible with aqueous systems and is recyclable. Compatibility defines the ability of the composition to impart adhesive properties to contiguous layers. Compatibility may be measured by various means such as viscosity stability, melt flow characteristics and cloud point. In particular, compatibility references the physical state wherein the elastomeric resin, tackifier and other additives are in a homogenous indistinct blend. In an incompatible adhesive, the elastomer, tackifier and other additives are in discrete areas in the molten state.